Climate whiplash in California: too much to bear, too little to handle?

Inter- and intra-annual water availability is naturally highly variable in Mediterranean regions, with swings between extremes costing nations potentially billions of dollars in damages and threatening lives. In California, future projections foresee an increase in the bimodal distribution of hydrological extremes, leading to greater hydroclimatic whiplash. Here, we quantify the relative impact of hydroclimatic whiplash on hydropower systems, flood control and water deliveries in the Central Sierra Nevada, California. We aim to explore at what point these services become less resilient to drought, and if wet whiplash years can re-establish an ‘average’ system state. To represent a wide range of wet, dry and dry-to-wet transitions, we sampled water years from upper (floods) and lower (droughts) quintiles, with replacement, across 30 years of future streamflow projections (2030-2060) from 10 global circulation models. Synthetic hydrological sequences of 2 to 5 dry years, followed by 1 to 2 wet years form a total sample of 200 whiplash sequences for the Stanislaus, Tuolumne, Merced and Upper San Joaquin River basins. This stress test indicates that the intensification of whiplash cycles would seriously challenge existing hydropower production, water storage and flood control operating rules. Compared to baseline averages, all basins had negative impacts on hydropower generation, with losses varying from 6% in the Merced to almost around 67% in the Upper San Joaquin, depending on the whiplash sequence. Agricultural and/or urban demands are most impacted in the Tuolumne and the Upper San Joaquin, in particular for all sequences. Historically, this basin has had more than 70% of outflows delivered to irrigation districts, therefore whiplash sequences tend to disrupt these services more easily. Meanwhile, carryover storage is negatively affected in all basins, but more noticeably in the Merced and Stanislaus basins, with losses of 7-60% and 15-31%, respectively, due to their small overall storage capacity. The small reservoirs in the upper watersheds and inflexible operating flood control rules constitute a challenge to accommodate whiplash impacts in the region. These results show heterogenous sensitivities of flood control releases, environmental flows and agricultural/urban deliveries with projected climate whiplash conditions, with varying degrees of annual, time and volumetric reliability. These services compete for scarce water supply within the low-elevation terminal dams in each basin. This analysis identifies perspectives on the challenges and risks of regional climate whiplash effects and adaptation strategies to include extremes and their impacts on water allocation to human and environmental purposes.